The invention relates to air valves and other air flow control mechanisms of the type used in air flow ducts such as those employed in heating and air conditioning systems. More particularly, the invention relates to air flow control mechanisms having at least one moveable blade, the blade having a surface sealable against an adjacent surface when the air valve or air control mechanism is closed.
In air valves and other air flow control mechanisms of the type used in air conditioning and heating ducts and which include at least one pivotable or moveable blade, when the blade is moved to an air flow restricting position and when the static pressure in the ducts is relatively high, air flow between the blade surface and an adjacent surface can result in noise in the form of a high pitched, pure tone such as a whistle. Such tones are usually caused when the gap or space between the blade and the adjacent surface approaches zero. The noise results when the air passes between two closely adjacent smooth surfaces and when those surfaces permit air to vibrate therebetween at a resonant frequency, thereby generating a standing wave between the adjacent surfaces, the standing wave providing the audible high frequency pitch or tone. While such audible tones or whistles are produced by smooth flat surfaces, similar audible tones are also produced if the adjacent surfaces are defined by sponge or open-cell foam gasketing material or if they are formed by vinyl extrusions or the like.
Examples of prior art air flow control mechanisms are shown in the U.S. Foster Patent No. 3,366,141; the U.S. Foster Pat. No. 3,495,521; the U.S. McCabe Pat. No. 3,908,529; and the U.S. Graham Pat. No. 4,038,781.